1. Field of the Invention
The present invention relates to a filter element which cleans an exhausted gas. Particularly, the present invention relates to a filter element which effectively collects particulates entrained with a gas exhausted from a Diesel engine and which is also regenerable.
2. Description of the Related Art
In order to remove particulates from an exhausted gas entraining them from a Diesel engine, a heat-resistant filter element is generally located at an intermediate portion of an exhaust pipe. When such a filter element collects a given amount of the particulates, the element is required to be regenerated since plugging of the filter element due to the collected particulates increases a pressure drop across the element so that a power output from the engine is decreased. Therefore, the filter element should be regenerated at a predetermined interval.
For such a filter element, a monolithic ceramic, a ceramic foam, a metal mesh and the like are used.
However, the filter element comprising the monolithic ceramic has a problem that cracking and/or volume loss of the filter element occur due to localized heating during burning off the collected particulates for regeneration. On the other hand, the filter element comprising the ceramic foam has an insufficient collection efficiency of the particulates. Further, the filter element comprising the metal mesh is damaged because of insufficient heat resistance when it is heated so as to burn off the collected particulates.
Japanese Patent Kokai Publication No. 3-275110 (corresponding to U.S. Pat. No. 5,194,078), disclosure of which is incorporated herein by reference, discloses a ceramic filter element in the form of a corrugated honeycomb construction in which two ceramic sheets each comprises chopped aluminosilicate fibers into a length of 0.1 to 10 mm and ceramic powder, and one of the sheets is corrugated and bonded to the other planar sheet to have a composite, and such a composite is spirally wound up or a predetermined number of the composites are laminated followed by firing the composite to have an integral one.
Japanese Patent Kokai Publication No. 2-256812 (corresponding to EP-A2-0 358 522), disclosure of which is incorporated herein by reference, discloses a Diesel particulate trap comprising, in addition to electrically heating elements, ceramic fiber filter material layers and gas permeable support layers which are laminated alternately. The Publication specifically describes an embodiment in which the ceramic fibers are of alumina-boria-silica ceramic.
In the filter element described in Japanese Patent Kokai Publication No. 3-275110, its mechanical strength is not sufficient since the filter material comprises the chopped fibers and the ceramic powder. As a result, the filter element may be damaged during its use. In addition, since the fibers and the ceramic powder are not uniformly dispersed, a size distribution of an opening of the filter element is likely to be non-uniform.
In the trap described in Japanese Patent Kokai Publication No. 2-256812, a collection efficiency of the particles is high since the trap comprises continuous ceramic fibers. However, when such a trap is used so as to collect the particulates from the exhausted gas of the Diesel engine over an extended period, the fibers are broken so that the trap cannot be used any more.